Voice loss is universal throughout the world, irrespective of age, gender, or social stratification and has a negative impact on effectiveness at work, in addition to being detrimental to psychosocial health. The importance of a reliable human voice has become increasingly critical in our age of communication. A healthy voice will likely become even more crucial in the 21st century; presently, greater than 80 percent of jobs in the United States are communication-based. A vocal deficit can be extremely disabling, and this will be more evident as voice-recognition becomes a driver for many information and communication technologies, i.e., replacing manual inputting (typing). Haxer, M., Guinn, L., and Hogikyan, N., Use of speech recognition software: A vocal endurance test for the new millennium? Journal of Voice, 15: 231-236 (2001). Furthermore, because of the unique nature of vocal performance, singing and/or oration are revered in a majority of primitive and modern societies. This is illustrated by the veneration ascribed to the religious leader, educator, entertainer, and at times the politician. Zeitels, S. M., Healy, G. B., Laryngology and Phonosurgery. New England Journal of Medicine, 349(9):882-92 (2003).
Optimal voice (laryngeal) sound production requires apposition of the vocal fold (cord) edges (glottal valve), which are driven into entrained oscillation by the sustained subglottal aerodynamic pressure and air flow from the tracheo-bronchial tree (FIG. 1A). The actual sound (acoustic signal) of the voice is produced by the air pulses that are emitted as the vocal folds open and close the glottis (opening between the vocal folds) during vibration. Ideal entrained vibration requires smooth vocal edges which close evenly, and which retain supple pliability. The vocal fold edges are covered by mucous membrane (mucosa), which are comprised of an outer epithelium and a superficial lamina propria (SLP), which lies just under the epithelium as shown in FIG. 1B. The epithelium has negligible rheologic properties and assumes the vibratory characteristics of whatever material lies beneath. Normal vocal fold vibration is manifested primarily as a wave of displaced mucosal tissue (SLP and epithelium) on the surface of the vocal folds, i.e., the mucosal wave. Presence of an intact mucosal wave is a primary sign of normal vocal fold structure and function. Since the SLP accounts for a majority of vocal fold vibration, loss of pliability of this layer due to the formation of stiff fibrosis or scar causes deterioration in vibratory function and associated hoarse voice (dysphonia). Laryngeal stroboscopy and high-speed videoendoscopy allows for clinical assessment of phonatory-mucosal vocal-fold vibration/oscillation and thereby assess the biomechanical behavior of phonatory mucosal layered microstructure, epithelium, and superficial lamina propria.
Voice production is optimal when the phonatory mucosa of both vocal folds retains favorable biomechanical/rheologic properties including elasticity and viscosity. This allows for efficient translation of the power source (aerodynamic pressure and flow) into an acoustic signal (voice). In a normal phonatory system, the vocal folds (glottis) are the sound source, while the pharynx, oral cavity, and nose function as a complex supraglottal resonating chamber, which individualizes a human's vocal signature.
From the initial cries at birth, through one's final words, the typical collision forces and shearing stresses sustained by the phonatory mucosa of vocal folds through life probably comprise the most substantial long-term soft-tissue trauma in the human body. A majority of the cases of untreatable hoarseness are due to diminished pliability of phonatory mucosa. There are likely more than 5 million individuals in the United States with this problem at any given time. However, the largest majority will never seek care and consider their vocal dysfunction to be their vocal signature/variation, because it is so commonplace and there is no remedy for this vocal insufficiency. This mucosal deficit is even incorrectly considered to be a normal component of the aging voice. Ironically, this dysfunctional mucosal soft tissue is often the result of decades of voice use (long-term trauma) rather than intrinsic age-related senescent tissue deterioration. Essentially, humans accumulate vocal mileage resulting in phonatory mucosal soft-tissue trauma during their activities in life. Those who are effusive and/or have vocally-demanding lives are prone to wear out and injure the phonatory tissues more rapidly. Given 21st century voice requirements, phonatory mucosal stiffness is increasingly impairing and terminating the career of voice professionals such as teachers, managers, executives, politicians, and performing artists.
Impliable (stiff) phonatory mucosa is also often associated with a variety of lesions such as polyps, cancer, nodules, and cysts, and vocal-fold membranes with these disorders are referred to as being “scarred.” Scarred phonatory mucosa can also result from prolonged endotracheal intubation, as well as from the treatment of carcinoma (surgery or radiation) or laryngotracheal stenosis. There is a large population of adolescent and young adults who have undergone airway reconstruction as infants or children. These elegant procedures that were designed in the 1970s, and modified in the 1980s, have allowed these children to function without an artificial airway. However, a majority of them have some type of vocal dysfunction. Smith, M. E., et al., Voice problems after pediatric laryngotracheal reconstruction: videolaryngostroboscopic, acoustic, and perceptual assessment. Int J Pediatr Otorhinolaryngol, 25(1-3):173-81 (1993). This dysfunction is typically the result of the unavoidable placement of life-preserving artificial airways and the subsequent reconstructive airway procedures.